Multiple beam optical memory system with solid-state lasers

ABSTRACT

An optical memory system employing multiple reading/writing optical beams from solid state lasers for simultaneously reading from a writing to multiple tracks of optical media to allow reading/writing of closely spaced adjacent tracks, a number a feature are disclosed. These include using vertical writing surface emitting lasers forming an array of beam sources, using lenslets associated with array, modulating the beams, and providing various optical elements and combinations of optical elements to compensate for beam and system imperfections.

This is a continuation of application Ser. No. 08/019,141, filed Feb.17, 1993, now U.S. Pat. No. 5,483,511.

A commonly assigned, concurrently-filed application, Ser. No.08/018,943, entitled "Multiple Beam Optical Memory System".

This invention relates to optical memories, and in particular to anoptical memory system in which multiple beams simultaneously readinformation from or write information to multiple tracks of movablestorage media and in which readout is performed by an array ofdetectors.

BACKGROUND OF THE INVENTION

Optical memory has been very successful in certain areas, the mostprominent being the Compact Disk (CD) involving playback (read only) ofmusical information. Although rewriteable optical media is beingdeveloped, for example phase-change and magneto-optical, othercharacteristics of optical recording technology have limited its use forcomputer related applications. One limitation is the rate at which datais read. This rate is limited by the spinning speed of the disk and thefact that only one source/detector is used. The other limitation isaccess time, or the average time it takes to access a randomly locatedbit of information. This time is limited by the mechanical motion of thehead over large distances along the radius of the disk.

Increase of data rate has been the object of many efforts. One approach,to speed up the rotational rate of the disk, is limited in the fact thatthe disks already rotate at almost their maximum practical speed. Use ofa shorter wavelength source will increase the data rate for a givenrotational rate. A factor of 2 reduction in wavelength will increase theareal data density by 4 times; however the linear density, which isrelevant to data rate, is only increased 2 times. To accomplish eventhis modest increase requires development of diode lasers emitting inthe near ultraviolet, which will take considerable time.

Use of multiple read sources can increase the data rate by an order ofmagnitude or more, independently of the other two approaches. Multiplelaser sources envisioned thus far have most often been linear, i.e.,one-dimensional (1D), arrays of edge-emitting laser diodes. See, forexample, the article by Carlin in Laser Focus World, July 1992, pp.77-84, and by Marchant in "Optical Recording" (Addison-Wesley, ReadingMass., 1990), both relating to optical disks, and by Bouldin andDrexler, U.S. Pat. No. 4,884,260, relating to optical tape. Thesesystems have the disadvantage of astigmatic elliptical beams resultingfrom the use of edge-emitting laser diodes. Correcting such beams in anarray is difficult. The edge-emitting laser diode geometry also does notallow the use of two-dimensional (2D) arrays, except by splitting thebeams by, for example, diffraction gratings. MacAnally in U.S. Pat. No.4,982,395 describes a composite optical grating which allows thesimultaneous reading of 2 adjacent concentric tracks. Marchant alsodescribes experiments conducted with a gas laser using a diffractiongrating to produce 9 beams. But this was not a practical system becauseof the size and the difficulties of firming, aligning, modulating andmaintaining of these beams. Moreover, the spacing between the focussedspots was too large to use with conventional CD media. The tworeferenced publications and the two referenced patents are hereinincorporated by reference. None of the prior art known to us describes apractical 2D readout from optical recording media, nor does it describea practical means for demagnification to make a 1D readout from with 4or more beams.

SUMMARY OF INVENTION

An object of the invention is an optical memory system providing highdata transfer rates and short access time.

A further object of the invention is an improved optical memory systememploying multiple beams for simultaneous multiple track reading orwriting.

Still another object of the invention is an optical memory systemproviding multiple beam readout of plural adjacent tracks with minimumcrosstalk.

In accordance with one aspect of our invention, we provide in an opticalmemory system one or more vertical-cavity surface-emitting lasers(VCSEL) providing plural optical beams capable of being focussed onadjacent tracks of the optical media for simultaneous reading of thedata incorporated in said adjacent tracks, or writing of data toadjacent tracks.

This aspect of our invention is based on the recognition that VCSELstypically generate circular, astigmatism-free beams, and are easilyfabricated in 1D or 2D arrays of beams. As a result, by incorporatingVCSELs in the system, a number of significant are achieved:

(1) 1D and 2D arrays with 4 or more lasers in a variety of arraygeometries and capable of reading from or writing to simultaneously 4 ormore tracks are easily obtained.

(2) The individual laser elements in the array can be spaced apartrelatively wide distances, thus simplifying fabrication, with a simpleoptical system provided to focus the multiple beams at the media toproduce optical spots with the very close spacings required toread/write adjacent media tracks.

We prefer to form the multiple beams using multipleindependently-addressable lasers. In such a case, the beams can bemodulated with different frequencies to help reduce crosstalk, with thedetector elements in a reading system provided with appropriateelectronics to filter out the modulating frequencies.

In accordance with another aspect of the invention, the optical memorysystem employs a 1D array source of light beams forming a linear arrayof closely-spaced read or write spots at the optical storage media. Inorder to realize close spacing of the read/write spots as would berequired for reading from or writing to adjacent closely-spaced tracksof the media, each of the light beams is associated with a lenslet atthe beam source. Preferably, the lenslets are integrated with theirrespective laser source. The provision of the lenslets not only providesufficient demagnification of the array of beams, so that they can bemore widely spaced at their source thereby greatly simplifyingfabrication, but also provide additional means to compensate for variousbeam aberrations or distortions to improve beam focusing and tracking atthe media.

In accordance with still another aspect of the invention, the lightbeams from their source comprise a 2D array of at least 4 beams. This,again, offers the benefits of allowing wider spacing at the source wherethe beams are generated or formed, yet providing closely-spacedread/write spots at the media. In addition, this geometry also allowscompensation for beam distortions to insure proper beam focussing andtracking out the media.

In accordance with still another aspect of the invention, an opticalsystem is provided for causing the multiple beams to focus on and trackmultiple traces of the media. A feature is the incorporation in the headof means for compensating for certain inherent optical defects whichmake it extremely difficult to form on the media closely-spaced focussedspots that retain their focussed condition and spacing as the head scansalong the tracks.

The above and further objects, details and advantages of the presentinvention will become apparent from the following detailed descriptionof preferred embodiments thereof, when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 schematically illustrate various beam spot arrays on opticalmedia for multiple reading/writing in accordance with the invention;

FIG. 5 schematically illustrates one way in accordance with theinvention to form multiple reading/writing spots;

FIG. 6 illustrates continuous scanning of multiple tracks on relatingmedia;

FIG. 7 is a schematic view of one form of optical system in accordancewith the invention for reading multiple tracks;

FIG. 7A schematically illustrates a modified optical system of theinvention;

FIG. 8 is a schematic view illustrating the optical relationship betweenmultiple beam sources and a composite lenslet system;

FIG. 9 shows schematically a typical detection array for use in a systemaccording to the invention;

FIGS. 10A and 10B show respectively in top and side views part of oneform of an array of VCSEL lasers and lenslets in accordance with theinvention;

FIG. 11 shows another embodiment of a system according to the inventionsimilar to FIG. 7;

FIG. 12 shows possible polarization characteristics of a systemaccording to the invention;

FIG. 13 illustrates a part of a modified optical system in accordancewith the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Laser arrays for various purposes using VCSELs have been reported in theliterature. To improve performance, diffractive microlenslets wereintegrated into the semiconductor substrate in which the VCSELs werebuilt. That is to say, each emitted beam was individually focussed byits own lenslet.

In accordance with an aspect of our optical memory invention, we use a1D or 2D array of VCSELs imaged through a conventional (singlemacrolens) optical system. The reasoning is manyfold as follows. Opticaldisk systems typically have a 2 mm working distance from the lens to themedia. Since the focusing lens is of high numerical aperture (NA), itsdiameter must be a few mm also. Thus the concept of having an individualmicrolens for each laser must either have a very short working distanceor very large separations between lasers. The only way to achieve a longworking distance and small spacings is to image an array through asingle lens. For a given optical magnification and use of a single lens,a given number of elements is most effectively imaged when configured ina 2-D array. Otherwise the elements are very far off the optical axis.

A square arrangement of beam spots focussed on a media plane andproduced by an array of individual laser elements integrated onto acommon substrate is schematically illustrated in FIG. 1. The beam spotof each laser is shown as a round dot on media designated 8. Spots from16 VCSEL lasers are shown, referenced 10₁ --10₁. The horizontal linesshown represent schematically the tracks on the optical media 8 and arereferenced 12₀ -12₁₇, and are provided to illustrate the geometry of thespot array relative to the tracks. This geometry we denote as a squarearrangement. To simplify the illustrations, the same reference numbersrefer to the same elements, and to avoid excessive clutter, some of thereference numerals do not appear on all the figures. For example, thecomplete listing of track references only appears in FIGS. 3 and 4, butare the same for FIGS. 1 and 2. The square is tilted with respect to therecording tracks with an angle such that each adjacent element 10₁ -10₁₆addresses an adjacent track 12₁ -12₁₆. The first element of the array oflasers, on the following scan (not shown), addresses the next adjacenttrack 12₁₇. Some of the laser elements may be eliminated without losingcontinuity in addressing the recording tracks. For example, theuppermost 10₁ and/or lowermost 10₁₆ elements can be eliminated. Someentire rows can also be eliminated to form a rectangular-shaped array. Avery similar oblique arrangement of laser spots 10₁ -10₁₆ can also beconstructed as shown in FIG. 2. A special case of the oblique array canbe viewed as two parallel linear subarrays, with one subarray beingoffset from the other by one track pitch distance. This is shown in FIG.3, with the laser element spots again designated 10₁ -10₁₆, and thetrack pitch designated 17. Although the preferred embodiments addressadjacent tracks of the recording medium, in some cases it can beadvantageous to address tracks in a non-adjacent geometry.

A further feature of our invention is to implement a linear array withmany elements addressing adjacent tracks. This is illustrated in FIG. 4,with the laser element spots again designated 10₁ -10₁₆. The prior artis only able to achieve addressing of non-adjacent tracks, or addressingof only a few adjacent tracks, or addressing of many adjacent tracksonly through of complex, heavy and expensive focusing lenses. The priorart address adjacent tracks with array orientations nearly parallel tothe tracks, when more than 2 beams are used, rather than the nearlyperpendicular orientation shown in FIGS. 3 and 4. Note that, in FIG. 3,the 2-D array of spots comprises two adjacent vertical columns, witheach column extending substantially perpendicular to the tracks depictedhorizontally, whereas in FIG. 4 the two columns have been merged into a1D array also extending substantially perpendicular to the media tracks.

In accordance with another feature of our invention, we form an array ofoptical beams such that when they are focused onto the recording media,the spacing between spots is comparable to the spacing 17 between tracksof conventional recording media, e.g., 1.6 μm. Minimizing the spacingwill minimize the off-axis aberrations of the focusing lens, therebyallowing use of a simple, lightweight and inexpensive lens. Toaccomplish the small spot spacing in a practical way, we construct thelaser sources to have a comfortably large spacing, e.g., 32 μm, and havethe optical system demagnify the source array by a significant factor,e.g., 20 times. Such a large demagnification is accomplished in acompact and efficient manner by the use of a microlens array.

FIG. 5 schematically illustrates one such optical system in accordancewith the invention. A common substrate (not shown) supports a pluralityof individual VCSELs designated 20₁ -20₃, each separated by a spacingequal to DX. Each VCSEL is associated with a microlens 21₁ -21₃ whichessentially reduces the divergence of each laser beam, designated 22₁-22₃. A single focussing and collecting lens doublet 23, 24 is providedto focus the three beams at the media surface or plane indicated at 8 toform three closely spaced optical spots 26₁ -26₃ each spaced apart by adistance equal to (1/m)DX, where m is the demagnification factor whichis the inverse of the system magnification. In conventional opticalmemory optical systems (no microlenses), the magnification would beapproximately the ratio of the numerical apertures of the focusing lensand of the collecting lens. In conventional systems, the demagnificationfactor is only about 2-3. Since VCSELs emit lower divergence beams, thedemagnification factor would be about 5-10. When microlenses areemployed, however, the demagnification can be tailored to fit the systemneeds, because each microlens effectively transforms the numericalaperture of the emitted beam from its original value to virtually anydesired value. In FIG. 5, the microlenses 21 decrease the divergence(numerical aperture) of the emitted beams and therefore increase thedemagnification factor, e.g., from 5 to 20. The use of microlens arraysto modify the optical system magnification is applicable to all of thebeam array configurations discussed above (FIGS. 1-4). In the opticalsystem of FIG. 5, many important components are left out, e.g. beamsplitters and detectors, in order to illustrate more clearly themagnifying properties of the system.

The extremely small pitch of the focused beams as shown in FIGS. 3 and 4allow the arrays to be arranged substantially perpendicular to thetracks as shown. For these two configurations, especially the lineararray of FIG. 4, rotation of the image about a vertical axis(perpendicular to the emitting surface) can be employed to compensatefor small imperfections in the optical system magnification whichotherwise must be extremely precise. If there are 16 laser elements, forexample, then a magnification would need to be accurate to less than+0.8% for all elements to track to within +0.1 μm. Capability to rotatethe image either by rotating the optical source array or by rotating anoptical element in the imaging system can therefore greatly relax thetolerances in the optical system specifications. The rotational anglecan be adjusted in the manufacture and fixed, or it can be activelyadjusted. For the "more 2-dimensional" configurations of FIGS. 1 and 2,rotation of the beams cannot be used to compensate for magnificationerrors, but it might be necessary in order to have each beam bewell-aligned with a data track.

The optical recording medium 8 is movable, and may be a rotating opticaldisk in which case the curvature of the tracks over the array size isnegligible and the tracks can be considered to be straight and parallel.The arrays of FIGS. 3 and 4 can be aligned substantially along a radialdirection of the disk. See, for example, FIG. 6, with rectangle 25representing a laser array with 4 beams 25₁ -25₄. Alternatively, theoptical recording medium may be a moving optical tape 8 withsubstantially straight and parallel tracks, and the arrays of FIGS. 3and 4 can be aligned substantially along a direction across the shortdimension of the tape.

The array of laser spots are preferably produced using an array ofVCSELs with one VCSEL for each spot. This approach allows both readingand writing of data and for individual correction of power emitted fromthe laser beam elements.

The optimum data formats in the system of the invention differ frompresent standard formats based on the use of a single laser element. Thegeometries in FIGS. 1-4, by addressing adjacent tracks, is advantageouscompared to alternatives because it requires only one type of mechanicalscan. The simplest scan, which also provides a continuous flow of data,uses the spiral approach of standard single-laser formats; however thespiral must be as wide in tracks as the number of laser elementssimultaneously scanned. For the examples of FIGS. 1-4, the spiral widthwould be 16 tracks. If the lasers are fired simultaneously for writing,or are at least fired at the same average rate, then each track willread at the same rate. This property allows each track to be read eitheras part of a word or as an independent data stream, i.e., flexibility ismaximized.

The "wide spiral" scan pattern for 4-bit wide addressing is illustratedin FIG. 6. As will be observed, the tracks labelled Track 1-Track 4 onthe media designated 8 spiral in groups of 4 tracks. The darker line forTrack 1 is used merely to make it easier to see the groupings. Thegrouping or swath of Tracks 1-4 are accessed simultaneously, shown byrectangle 25. The dashed rectangle 30 shows a second position of thehead to access an inner swath of the tracks. The head 25 can moveuniformly inward to stay continuously on the tracks and thuscontinuously, without interruptions, read/write data on the tracks.

In a concentric, sectorized format, the scan would have a jump for eachdisk rotation, each jump traversing a number of tracks equal to thenumber of optical beams for the addressing. For the arrays of FIGS. 1-4,each jump would be 16 tracks. A small radially-oriented gap in the datamight be necessary to allow the jump to take place without data loss.For applications such as music or multimedia real-time display of thedata, these interruptions could be smoothed out by using modest-sizedbuffer data storage in the electronics. Each swath of tracks should havethe same number of bits in each track in order to simplify the software.Obviously, if the amount of data on the disk is maximized, a swath nearthe center of the disk will have fewer bits per rotation than a swathnear the disk's outer edge. It could be advantageous to have a number ofadjacent swaths have the same number of bits, with larger changes in thebit number occurring less frequently.

The use of memory buffers with appropriate electronics and software canallow the optical memory system to read or write in a variety offormats. For example, consider a conventional disk written in aconcentric format with sequential data arranged circularly around thedisk. The multiple beam system of this invention would read a plurality,e.g., 16, tracks in one rotation. Having memory buffers of sufficientsize to store one revolution of information (times the number of tracksread) would allow this information to be rearranged to whatever formatis necessary, e.g., corresponding to 16 rotations of a conventionalsingle-beam system. The inverse of this procedure could be employed towrite in alternative formats.

For small array sizes, the VCSEL array and detector array can almost bedirectly retrofitted into an existing optical head. As long as the arrayorientation and imaging system magnification are within tolerance, onlyone laser element of the array needs to undergo focusing and trackingadjustments. These are the standard focusing and tracking adjustmentsmade in a single-element system. See, for example, the description givenin Marchant, "Optical Recording" (referred to previously) of aconventional single-element system and the standard focusing andtracking mechanisms and adjustments, the contents of which are hereinincorporated by reference. It is likely that performing focusing andtracking on two elements would be advantageous. For the arrays of FIGS.3 and 4, this would allow automatic and rapid compensation for errors inmagnification or in track pitch by rotation of the beam array. Thetwo-element focusing would also automatically correct any tilt in thelong axis of the array.

An optical system similar to those in use for single laser memories butconstructed in accordance with the invention is shown in FIG. 7. In thisembodiment, an array 35 of VCSELs is employed. The individual beams areshown as a single beam 36 (made up of a plurality of individualbeams-not shown) to illustrate that optics similar to that used in theconventional single beam head can be used with the invention. (The beamsoverlap each other throughout most of the system as shown in FIG. 5). Amicrolens array (as previously described) combined with a fieldflattener 37 is also employed. In the beam path is the usualpolarization beamsplitter 38. The transmitted beams 39 are passedthrough a quarter-wave plate 39, a collecting lens 40, movable 40'axially along the beam axis for adjusting the magnification or focusing,an aperture stop 41, and a field lens 42, and is impinged on a mirror 43mounted within a conventional housing 44 movable laterally 45 forscanning and vertically 46 for focussing with respect to the medium 8.Also within the housing 44 is the usual focusing lens 47. The reflectedbeams, follow the same beam path, except that they are deflected at thesplitter 38 to a detector array 48. Preferably a second microlens array49, possibly with field flattening and perhaps other optical features,focuses the beams onto the detector array 48.

Though not shown, it will be appreciated that the detector array 48 willcomprise approximately the same number of discrete detector elements asthere are discrete beams, with most of the detector elements oriented inthe same manner as that of the laser elements in the array 35. At leastsome of the detector elements may be further subdivided into, forexample, a quadrant of detector subelements, for use in focusing andtracking.

FIG. 9 illustrates one possible arrangement of the detector elements forthe 2D array illustrated in FIG. 3. In this example, the array elementsare designated 48₁ -48₁₆. The end elements, 48₁ and 48₁₆, are subdividedin four separate subelements subscripted a-d, as is known per se, andconnected to known focussing and tracking circuitry (not shown) forprocessing the signals resulting from the location on the subelements ofthe return beam. Signal processing of the signals derived from each ofthe detector elements 48₁ -48₁₆ would be individually carried out. Thesystem can also include the usual detector elements for focusing andtracking associated with the center beam of the beam array as well aswith the two outermost beams of the beam array (FIG. 9), and preferablyintegrated with the detector array. The folding mirror 43 and focusinglens 47 are the only components which must move rapidly to accessvarious locations on the reading disk for disk systems. It is possibleto use standard components for the folding mirror 43 and focusing lens47, implementing any necessary added complexity in the non-moving partof the system. This design approach takes advantage of the increase indata rate by using the laser array 35, but does not compromise theaccess time of the system. Preferably, the focusing lens comprises nomore than two surfaces which deviate substantially from being flat. Thesystem in FIG. 7 employs the polarization beamsplitter 38 andquarter-wave plate 39 to attain high optical efficiency, if needed. Anadvantage of an efficient optical system is that it greatly decreasesthe power requirements for the lasers, which is especially important forarrays. A Faraday rotator can be substituted for the quarter-wave plate39.

Since the beam divergences from the source array 35 is quite small,especially if a microlens array 37 is used, it is possible to place thebeamsplitter of FIG. 7 between the source array 35 and the collectinglens 40 as shown. This arrangement makes the system more compact. Itwill also make feasible direct magnification adjustment by adjusting thedistance between the lasers 35 and the collecting lens 40, indicated bydouble arrow 50. This can be used to compensate for errors inmagnification or track pitch in all the geometries of FIGS. 1-4. In themore conventional arrangement, which places the beamsplitter after thecollecting lens, varying the distance between the laser array andcollecting lens affects the focusing of the beams onto the detectorarray and thus causes focusing errors on the recording medium. In thearrangement of FIG. 7, longitudinal movement of the collecting lens 40,shown by arrows 40', can be used for magnification adjustment becausethe positions of the laser array 35 and detector array 48 are held fixedwith respect to the beamsplitter 38. Thus, when the optical head whichincorporates all the elements shown is manufactured such that focusingof the beams onto the detector array 48 corresponds to having the beamsfocused on the recording medium 8, the condition is preserved despitemagnification adjustments.

Since the optical memory system employs an array of detectors forreading the information, there is the possibility that signals from onetrack will fall onto a detector element corresponding to another track.One means to eliminate this crosstalk between channels is to modulatethe sources at different frequencies and to have the correspondingdetector elements filter out these frequencies. The difference infrequencies between channels should be larger than the frequency atwhich information bits pass by. For example, for a given channel theinformation bits might be read about once every microsecond. One wouldthen want about a 10 MHz difference between channel frequencies. A 2×2array of 4 channels could have modulation frequencies of 100, 110, 120and 130 MHz to have sufficient bandwidth separation and keep within acomfortable frequency range of the electronics. If there are more than 4channels, redundancy in frequency is allowable so long as two channelswith the same frequency modulation are not adjacent. For thetwo-dimensional geometries of FIGS. 1-3, four different frequenciesshould be sufficient; a one-dimensional array (FIG. 4) could use aslittle as two frequencies. Modulating the lasers at these frequencieshas the added benefit of greatly reducing the effects of opticalfeedback into the lasers (Marchant, p.153).

This is illustrated in the modified system illustrated in FIG. 11, whichshows a modulator 60 comprising appropriate driving circuitry formodulating the VCSEL array so as to generate thedifferent-frequency-modulated light beams. Processing means includingfilters for the modulated signal frequencies is shown at 62 for theupper detector array 48.

FIG. 11 also shows at the bottom side of the splitter 38 a focusing lens63 and a second detector array 64. The latter can be conveniently usedto monitor the laser array 35, for, for example, adjusting the drive forthe individual laser elements to ensure they are generatingsubstantially constant beam powers. The remaining elements shown in thesystem are the same as in FIG. 7. FIG. 11 also shows by the dashed linereferenced 80 a rigid mechanical interconnection between the laser array35, lens array 37, first detector array 48, monitor lens 63 and monitorarray 62, quarter-wave plate 39, and the beam splitter 38, to preservefocussing conditions despite magnification adjustments. The optical axisof the system is represented by the central ray referenced 34.

While the invention is especially adapted for use with movable opticalmedia having closely-spaced tracks, typically 2 μm or less, because ofthe incorporation of a number of features of the invention, which can beused separately or together, designed to maintain the spots focussed andto properly tracking adjacent media tracks, the invention is not limitedto such applications. Moreover, in the preferred embodiments it ispreferred that the head scan along a line generally transverse to thetracks, e.g., along a radius of a rotating disk, or across an elongatedtape.

Since standard CD disk system lenses have good correction of sphericalaberration, coma and astigmatism within a field diameter of about 50 μmto 100 μm , small arrays for example, up to 75 elements, can probablyuse the standard CD lenses. For a 1.6 μm track pitch, the 16-elementarrays of FIGS. 1 and 2 have field diameters of 28 μm, while those ofFIGS. 3 and 4 have 24 μm field diameters. Beam walkoff, field curvatureand distortion could cause some problems however. Since the collimatedbeams 52 emerging from the collecting lens 40 propagate in differentdirections, at some distance they will no longer overlap sufficiently tobe focused by the focusing lens 47. This may or may not be a problem inthe optical memory system. The standard optical approach to this problemis to use a field lens 42 (FIG. 7). The effect of refraction by fieldlens 42 is not shown. The field lens in this case would typically imagethe collecting lens 40 onto the focusing lens 47. Thus the degree ofoverlap at the collecting lens (before most of the walkoff occurs) wouldbe transferred to the focusing lens, whatever the separation is betweenthem. For an optical memory system, a different optimization ispreferred. Complete overlap of the discrete optical beams does notactually occur at the collecting lens 40 (see FIG. 5) but in its focalplane opposite the laser array. Thus it is more appropriate for thefield lens 42 to image this plane onto the focusing lens 47 to centerall the beams on the focusing lens. Furthermore it may be preferable tomake the beams overlap in the front focal plane of the focusing lens 47.In this case the system is said to be "telecentric" and has theadvantage in that all of the focused beams strike the recording mediumat normal incidence. The actual preferred use (or non-use) of the fieldlens will depend on the required system performance and constraints suchas size and cost.

Field curvature could be corrected by a field flattener, shown at 37,preferably located close to the VCSEL array 35. Traditional fieldflatteners for optical imaging systems are essentially very low powerlenses located as closely as possible to the object or image plane,preferably the latter. For the optical memory system the field flatteneris most conveniently located near the source array. If a microlens arrayis used, it is preferable to integrate the field flattener with themicrolens array, shown at 37, since they can be mass-produced as asingle monolithic unit at extremely low cost by injection molding. Thereare at least several ways in which a field flattener can be integratedinto the system. First, the focal lengths of the microlens can be variedappropriately across the array. In other word, the different positionsof the laser source relative to the optical axis can be compensated byconfiguring the associated microlenses to have different focal lengths.A more attractive way is to replace the flat surface of the microlensarray with a field flattening surface. The surface could have acontinuously curved surface as in traditional field flatteners.Alternatively, since the laser array represents a small number ofdiscrete objects, a segmented approach can be applied in which differentsurfaces positioned differently along the optical axis can focus themultiple beams at an approximately planar surface.

The segmented approach is illustrated in FIG. 8. A monolithic piece 37containing the microlens array 55 is typically formed of plastic orglass and is shaped into segments 56 as shown on the side facing thesources 35 and refracts the light rays in accordance with well knownlaws of refraction. This creates virtual sources 57, as "seen" by themicrolens which are shown in the figure to have a curved arrangement.Thus a focusing lens 47 having the normal sign of field curvature wouldfocus the spots in a plane. Typically, the segment surfaces 56 would beoriented at substantially different angles as required, with the anglestypically varying in the range of about 0.002°-0.1° . A given fieldflattener would have characteristics optimally matched to a particularoptical imaging system and would not necessarily be sufficientlyaccurate if, for example, the focusing lens was substituted by anotherone of different design. For optical systems not using a microlens arrayit is possible to design the collecting lens 40 to have "negative" fieldcurvature to cancel the effects of the focusing lens, although this willadd expense and complexity to the lens. In the preferred arrangement,the stepped surfaces 56 would be displaced from one another in adirection substantially perpendicular to the optical axis (vertically inFIG. 8) by more than two optical wavelengths for best performance.

It is also possible to vary the diameters of the laser apertures whenVCSELs are used. This approach is rather limited, however, and links thelaser arrays to a particular optical system's characteristics, anundesirable feature. If the optical system includes an intermediateimage plane (not shown in FIG. 7) then a field flattener can be placednear that image plane.

It is also possible to use a "staircase lens" to modify field curvature,distortion, and chromatic aberration. See, for example, the descriptiongiven in Sasian and Chipman, Applied Optics, Vol. 32, No. 1, 1 Jan.1993, pages 60-66, whose contents are herein incorporated by reference.The staircase lens could then be placed near the collecting lens 40, thefield lens 42, or the focusing lens 47. It is even possible to integratea staircase feature directly onto one or more of the surfaces of theaforementioned lens elements. FIG. 13 illustrates one possibility, witha staircase lens 90 located before the collecting lens 40, with thelines referenced 54, representing several beams of the array.

Just as field curvature can render it difficult or impossible to focusall elements simultaneously, optical distortion can make it difficult orimpossible for all elements to track simultaneously. "Pre-distortion" ofthe laser array is one approach to compensate for the optical systemdistortion; however it links a particular laser array pattern with aparticular system. Preferably, the field flattener of FIG. 8 can also bemodified to predistort the virtual sources by tilting the flat surfaces56 facing the sources, as indicated by arrows 58 each at its appropriateangle. Alternatively, the arrangement of the microlens elements in themicrolens array can be distorted, thus predistorting the arrangement ofthe virtual sources such that the image through the optical system hasthe desired arrangement. Either of the approaches adds no additionalcomplexity to the system. Another means for dealing with distortion isuse of the aperture stop 41. Moving the aperture stop 41 (shown by arrow51) will affect the distortion as will moving the field lens (shown byarrow 51). Distortion correction and correction for beam walkoff areprimary motivations for use of the field lens. The need for distortioncorrection would take priority over the desire for telecentricity in thesystem. The field lens can also act simultaneously as the field stop,simplifying the system but constraining the positions of the lens andstop to coincide.

FIG. 7A shows a modified optical system of the invention from the sourcearray 35 to the quarter-wave plate 39. The remainder of the system, notshown, would be the same. In the modified system, a second quarter-waveplate 39' is provided before the beam splitter 38, and a second detectorarray 64 with a preceding detector lens 63 (see FIG. 11) is incorporatedin the system. Also, the additional microlens array and field flattener49 is incorporated before the first detector array.

The polarization characteristics of the source array also affect theoptical system layout. The preferred case is when the sources are alllinearly polarized with the same orientation. Then the configuration ofFIG. 7 (without the first quarter-wave plate 39) is used with highoptical efficiency and minimal optical feedback into the sources. It iswell known that VCSELS, when operating in the lowest order mode, arelinearly polarized. Although most of the VCSELs are polarized along onecrystal axis, a significant number of them are polarized in theorthogonal direction, and a few may be polarized at random orientations.For the case when the VCSEL polarizations are confined to two knownorthogonal directions, the quarter-wave plate 39' between the microlensarray 35 and the beamsplitter 38 of FIG. 7A can be oriented to make allthe beams circularly polarized, with one polarization left-handed andthe other polarization right-handed circular. One half of the power fromall the beams is then reflected by the polarization beamsplitter 38. Theother half continues on to the medium. The system efficiency is then onehalf as high as the one having identically polarized sources, and thesources are still isolated from optical feedback. This is illustrated inFIG. 12. The horizontal line 82 represents the optical axis, with thebeams moving to the right. The circles represent polarizations of a beamat different positions in the system. The circles, normally transverseto the beam, have been rotated to face frontward for clarity. The firstcircle 83 illustrates a beam with one orthogonal direction ofpolarization before the quarter-wave plate 39', which has its axisoriented 45 degrees off the light polarization direction, illustrated at84, to make the light circularly polarized. Circularly polarized lightcan also be represented as two orthogonal linear polarizations. Thisrepresentation is used to show the polarization at the beamsplitter 38and is indicated at 85. The reflected half is illustrated at 87, and thetransmitted half at 86. If the beam at 83 had the other orthogonalpolarization direction, the system would work the same way. The sameeffect can also be accomplished via a halfwave plate oriented 22.5degrees from either of the orthogonal light polarization directions.

The same system efficiency and feedback isolation could be obtainedwithout the additional quarter-wave plate 39' by orienting the VCSELarray 35 and the polarization beamsplitter 38 such that all the VCSELbeam polarizations are oriented at 45 degrees with respect to thebeamsplitter. The same performance could also be obtained if the VCSELbeams are unpolarized, having two longitudinal modes of nearly equalpower and orthogonal polarizations, or are circularly polarized, or havethe polarizations rotates at a rate much faster than the rate at whichdata is read by each beam.

The case where the VCSEL beams are all polarized but in constant randomdirections is more difficult to handle. One approach uses the system ofFIG. 7 without the quarter-wave plates and with the beamsplitter beinginsensitive to the polarization. This system is one fourth as efficientas the one having identically polarized sources, and the sources are notwell isolated from optical feedback. An alternate approach to handlingbeams which are polarized in constant random directions is to rotateeach beam polarization to the desired orientation. One way to accomplishthis is by an array of half-wave plates, each of which is rotated to theproper orientation to rotate the polarization of a particular beam asdesired. Another way uses optically-active crystals, each of the properlength to rotate the polarization as desired. The half-wave plates oroptically active crystals would be placed in the same location as thefirst quarter-wave plate 39 of FIG. 7.

The benefits of the invention are mainly achieved when the number ofdiscrete beams varies between about 4 and 100. The smaller numbersimplifies the focussing requirements and optics. The larger numberresults in faster data transmission.

FIG. 9 showed schematically a typical detector array for use with alinear array of 16 laser beams. In this embodiment, two of the detectorelements 48₁ and 48₁₆ are subdivided into quadrant detectors to allowthem to yield focusing and tracking information by the astigmatic methodas described by Marchant. For the astigamtic focusing, at least thebeams impinging on the quadrant detectors should have astigmatismintroduced between the beamsplitter 38 and the detector array 48 of FIG.7.

FIG. 10A shows in a top view part of a typical VCSEL array that can beused with the invention, and FIG. 10B shows in a side view an array oflower divergence laser beams generated when the VCSEL array isassociated with a lenslet array. More specifically, this array, assuming16 lasers were present, will generate the spot pattern shown in FIG. 3,and each laser element shown 35₁ -35₉, is powered by a separate driver,only four of which are shown, 80₂, 80₄, 80₆, 80₈. FIG. 10B shows a sideview of the laser array 35 with integrated microlenses 82 which reducethe divergences of the beams in the same fashions as the microlens arrayof FIG. 5.

It is advantageous for all the elements to the left of the collectinglens 40 in FIG. 11 to be rigidly mounted as a single unit. Thequarter-wave plate 39 can be cemented to the beamsplitter 38. Themicrolens array 37, if present, can be manufactured with fixtures tomake easy alignment with the beamsplitter 38 on one side and the laseror detector array on the other side. The laser and detector arrays couldalso be manufactured with fittings to complement the fixtures on themicrolens arrays. An example of this is projections from the microlensarrays (easy to manufacture especially if injection molding is used)which fit into holes etched into the laser or detector arrays. Similarapproaches could be used for the detector lens and a second detectorarray if they are included. This kind of mounting technique providesmanufacturability at low cost and good mechanical stability. The sizesof the beamsplitter, quarter-wave plate and detector lens are minimizedwhich helps minimize costs. Furthermore it allows one to perform motionssuch as rotation of the laser array 35 about the axis 34, shown in FIG.11 by arrow 85, without requiring corresponding motions of the detectorarray(s). Such a rotation capability is desirable for alignment of asquare array or for magnification correction/compensation in a linear orquasi-linear array. In the rigid-mounted configuration the entire unitis rotated and all alignments are preserved.

VCSEL arrays integrated with lenslets are available commercially fromPhotonics Research Incorporated, of Boulder, Colo. In addition, thetechnology for making VCSEL arrays is well known. See the previouslyreferenced publications and patents, and the following for detaileddescriptions:

(a) Laser Focus World, May 1992, pgs. 217-223;

(b) Photonics Spectra, Nov. 1992, pgs. 126-130;

(c) Scientific American, Nov. 1991, pgs. 86-94;

(d) IEEE J. Quantum Electron., June 1991, pgs. 1332-1346;

(e) U.S. Pat. No. 4,999,842.

One of us is an author or inventor of each of the foregoing references.

Optical recording with single or multiple beams is described in:

(f) Laser Focus World, July 1992, pgs. 77-84;

(g) SPIE Vol. 1499 Optical Data Storage '91, pgs. 203-208;

(h) U.S. Pat. Nos. 4,982,395; 4,884,260; 4,712,887.

All of the foregoing referenced publications are incorporated herein byreference.

Although there have been described what are at present considered to bethe preferred embodiments of the invention, it will be understood thatthe invention may be embodied in other specific forms without departingfrom the essential characteristics thereof. The present embodiments aretherefore to be considered in all respects as illustrative, and notrestrictive. This scope of the invention is indicated by the appendedclaims rather than by the foregoing description.

What is claimed is:
 1. An optically aligned memory system comprising:adata storage medium comprising data arranged as substantially linear andparallel tracks, said storage medium further comprising opticalalignment features; a plurality of optical sources capable of generatinga plurality of optical beams propagating in paths of substantially thesame direction along an optical axis to said storage medium, saidplurality of light sources arranged in an array having at least a firstand second dimension; a plurality of detectors; a beam splitter disposedin said paths between said array of light sources and a collecting lens,said single collecting lens disposed in said paths between said array oflight sources and a focussing lens, said focussing lens disposed in saidpaths between said plurality of beams propagating through saidcollecting lens and said storage medium; wherein said plurality of beamsare reflected from said storage medium in reverse along said paths anddiverted to said plurality of detectors by said beam splitter such thateach of said detectors receives substantially one of said plurality ofreflected beams and emits an electronic signal in response thereto;means for processing the signals from two or more of said plurality ofdetectors to determine the existence or absence of said opticalalignment features; and a plurality of microlenses disposed in saidpaths between said plurality of light sources and said beam splitter tomodify the convergence or divergence of at least one of said pluralityof optical beams.
 2. An optically aligned memory system comprising:adata storage medium comprising data arranged as substantially linear andparallel tracks, said storage medium further comprising opticalalignment features; a plurality of optical sources capable of generatinga plurality of optical beams propagating in paths of substantially thesame direction along an optical axis to said storage medium, saidplurality of light sources arranged in an array having at least a firstand second dimension; a plurality of detectors; a beam splitter disposedin said paths between said array of light sources and a collecting lens,said single collecting lens disposed in said paths between said array oflight sources and a focussing lens, said focussing lens disposed in saidpaths between said plurality of beams propagating through saidcollecting lens and said storage medium; wherein said plurality of beamsare reflected from said storage medium in reverse along said paths anddiverted to said plurality of detectors by said beam splitter such thateach of said detectors receives substantially one of said plurality ofreflected beams and emits an electronic signal in response thereto;means for processing the signals from two or more of said plurality ofdetectors to determine the existence or absence of said opticalalignment features; wherein said optical beams are impinged on a mirror,disposed in said paths between said collecting lens and said focussinglens, said mirror being movable horizontally to track said plurality ofbeams on said storage medium.
 3. An optically aligned memory systemcomprising:a data storage medium comprising data arranged assubstantially linear and parallel tracks, said storage medium furthercomprising optical alignment features; a plurality of optical sourcescapable of generating a plurality of optical beams propagating in pathsof substantially the same direction along an optical axis to saidstorage medium, said plurality of light sources arranged in an arrayhaving at least a first and second dimension; a plurality of detectors;a beam splitter disposed in said paths between said array of lightsources and a collecting lens, said single collecting lens disposed insaid paths between said array of light sources and a focussing lens,said focussing lens disposed in said paths between said plurality ofbeams propagating through said collecting lens and said storage medium;wherein said plurality of beams are reflected from said storage mediumin reverse along said paths and diverted to said plurality of detectorsby said beam splitter such that each of said detectors receivessubstantially one of said plurality of reflected beams and emits anelectronic signal in response thereto; means for processing the signalsfrom two or more of said plurality of detectors to determine theexistence or absence of said optical alignment features; wherein saidplurality of optical sources comprises: an array of independentlyaddressable solid state lasers; andwherein said optical beam generatingmeans comprises: an array of vertical-cavity surface emitting lasers. 4.An optically aligned memory system comprising:a data storage mediumcomprising data arranged as substantially linear and parallel tracks,said storage medium further comprising optical alignment features; aplurality of optical sources capable of generating a plurality ofoptical beams propagating in paths of substantially the same directionalong an optical axis to said storage medium, said plurality of lightsources arranged in an array having at least a first and seconddimension; a plurality of detectors; a beam splitter disposed in saidpaths between said array of light sources and a collecting lens, saidsingle collecting lens disposed in said paths between said array oflight sources and a focussing lens, said focussing lens disposed in saidpaths between said plurality of beams propagating through saidcollecting lens and said storage medium; wherein said plurality of beamsare reflected from said storage medium in reverse along said paths anddiverted to said plurality of detectors by said beam splitter such thateach of said detectors receives substantially one of said plurality ofreflected beams and emits an electronic signal in response thereto;means for processing the signals from two or more of said plurality ofdetectors to determine the existence or absence of said opticalalignment features; and a second plurality of detectors, coupled to saidplurality of beams through said beam splitter, for monitoring outputpower of said plurality of light sources.
 5. An optically aligned memorysystem comprising:a data storage medium comprising data arranged assubstantially linear and parallel tracks, said storage medium furthercomprising optical alignment features; a plurality of optical sourcescapable of generating a plurality of optical beams propagating in pathsof substantially the same direction along an optical axis to saidstorage medium, said plurality of light sources arranged in an arrayhaving at least a first and second dimension; a plurality of detectors;a beam splitter disposed in said paths between said array of lightsources and a collecting lens, said single collecting lens disposed insaid paths between said array of light sources and a focussing lens,said focussing lens disposed in said paths between said plurality ofbeams propagating through said collecting lens and said storage medium;wherein said plurality of beams are reflected from said storage mediumin reverse along said paths and diverted to said plurality of detectorsby said beam splitter such that each of said detectors receivessubstantially one of said plurality of reflected beams and emits anelectronic signal in response thereto; means for processing the signalsfrom two or more of said plurality of detectors to determine theexistence or absence of said optical alignment features; and means forrotating said plurality of optical beams.